Manchester Me7Hoirs, Vol. Iv. {igii), No. ^. 3 



" the germs of most of the ideas which I have since 

 expounded more at length in different essays, and which 

 have been considered as discoveries of some importance. 

 For instance, the idea that steam or the vapour of water 

 is an independent elastic fluid .... and hence that all 

 elastic fluids, whether alone or mixed, exist indepen- 

 dently."' He was probably influenced most by Deluc 

 in forming this opinion, but other persons, including 

 Bryan Higgins and Pictet, had expressed views more or 

 less the same as Deluc's. 



Da/ ton's theory of mixed gases. 



Thus Dalton had early regarded the constitution of 

 mixed gases from the physical point of view. In the 

 year 1801 he formed a precise theory of his own, which 

 he explained and maintained publicly. The paper in 

 which he describes it, forms one of the set of four experi- 

 mental essays, which, Dalton himself said, " drew the 

 attention of most of the philosophers of Europe." 



He put his theory in the following way : " When two 

 elastic fluids, denoted by A and B, are mixed together, 

 their is no mutual repulsion amongst their particles, that 

 is, the particles of A do not repel those of B, as they do 

 one another." ' At first the doctrine was not understood, 

 and Dalton had to make further efforts to throw light 

 upon it. His hypothesis meant that while gaseous 

 particles of the same kind repelled one another, there 

 were no forces, whether of repulsion or attraction, 

 between particles of different kinds. Particles of one 

 kind could offer only a passive resistance to the motion of 

 another kind of particles, and acted only as temporary 

 obstacles, in the same way as the pebbles in a stream 



- " Meteorological Observation and Essays," 2nd Ed., p. v. (1834). 

 ^ Manchester Memoirs, [i], vol. 5. p. 536, 1802. 



